The present invention relates to a novel process for producing a naphthalene compound having 2-hydroxy-2-propyl group(s) on the naphthalene ring, that is, (2-hydroxy-2-propyl)naphthalene compound (hereinafter referred to as HPN).
HPN produced according to the present invention is easily convertible into a compound useful as a raw material for plastics, particularly as a raw material for engineering plastics, for instance, 2,6-naphthalenediol.
Recently, various polymers have been developed as engineering plastics, and among them, aromatic polyesters obtained by using naphthalene derivatives having hydroxy group(s) or carboxyl group(s) as raw material have attracted one's attention because of their excellent heat-resistance and mechanical strength. Accordingly, the development of an industrially profitable process for producing the naphthalene derivative having hydroxy group(s) or carboxyl group(s) which is used as the raw material of the aromatic polyesters has been demanded.
Since the naphthalene derivative having hydroxy group(s) or carboxyl group(s) can be easily produced from the HPN, for instance, 2,6-naphthalenediol is easily available in a high yield by oxidizing 2,6-di(2-hydroxy-2-propyl)naphthalene with hydrogen peroxide in acetonitrile or 1,4-dioxane in the presence of an inorganic acid (refer to Japanese Patent Application No. 60-123819 (1985)), the usefulness of HPN as an intermediate raw material has been raised.
As a process for converting an isopropyl group bonded to a naphthalene ring into a 2-hydroxy-2-propyl group by oxidation, a process which comprises oxidizing with molecular oxygen in the presence of an aqueous alkali solution, for example, Japanese Patent Publication No. 39-21242 (1964), has been proposed, and as a process for converting an isopropyl group bonded to a benzene ring into the 2-hydroxy-2-propyl group, a process has been proposed in Japanese Patent Publication No. 39-19355 (1964), Japanese Patent Application Laying-Open (KOKAI) No. 58-162539 (1983) and DE-OS No. 12 33 839. However, no process in which peroxodisulfate is used as an oxidizing agent has been known.
As has been described above, under the circumstances, any industrially profitable process for producing HPN has not been established and there are very few research reports on such a process despite of the increased industrial usefulness of HPN.
As a result of the present inventors' studies, it has been found by the present inventors that (1) a naphthalene compound having isopropyl group(s) on the naphthalene ring (isopropylnaphthalene compound) is easily oxidized into HPN in the presence of peroxodisulfate in a mixed solvent of water and an organic polar solvent, (2) the yield of HPN can be raised in the case where the oxidation is carried out in the presence of iron (II) salt and/or chloride and (3) in the case where a naphthalene compound having two or more isopropyl groups on the naphthalene ring is used as a starting material, an oxidation product of a different stage of oxidation can be selectively obtained by selecting a proper reaction conditions, and on the basis of the findings, the present inventors have accomplished the present invention.